The invention relates to a cathode ray tube comprising a funnel portion which is covered at least in part with an electrically conductive layer, and a deflection unit mounted on the funnel portion.
A cathode ray tube wherein the funnel portion is covered with an electrically conductive layer is known. JP-A 05-266829 describes a cathode ray tube display device wherein a conductive material, e.g. graphite, is provided on a funnel portion, and a deflection unit is mounted on the cathode ray tube. Generally, cathode ray tubes (CRTs) comprise a number of undesired radiation sources.
An important source of radiation is the deflection unit which comprises coils generating an alternating electric current. In this connection, a method has been proposed in which an electrically conductive layer, e.g. graphite, is provided on the surface of the funnel portion, which is connected to ground to electrostatically shield the alternating electric field.
During operation of the cathode ray tube, unwanted electric discharges may occur at the position of the deflection unit. This phenomenon, known as arcing, may damage the cathode ray tube and possibly reduce its lifetime. To avoid the harmful consequences of the arcing phenomenon, an insulating film between the deflection unit and the graphite layer on the tube is commonly applied. However, the insulating film is an additional layer or component, which raises the costs of the cathode ray tube.
It is an object of the invention to provide a cathode ray tube wherein alternating electric fields generated by a deflection coil unit are shielded with an electrically conductive layer, which reduces the need of an additional insulating film.
The cathode ray tube according to the invention is characterized in that the electrically conductive layer comprises an electrically conductive polymer. The inventors have found that an additional insulating film may be omitted by applying an electrically conductive polymer.
The embodiment of claim 2 provides a class of electrically conductive polymers which have the additional advantage of being relatively cheap and easy to apply.
The embodiment of claim 3 provides an electrically conductive polymer which has the advantage of possessing self-healing properties when applied as a layer between the deflection unit and the funnel portion. In this respect, self-healing means that, if the arcing phenomenon occurs at a certain location during operation of the cathode ray tube, the properties of the polymer layer are changed at that particular location. In particular, locally the polymer is not electrically conductive anymore. Consequently, possible arcing phenomena do not lead to significant damage of the cathode ray tube.
The embodiment of claim 4 provides an electrically conductive layer having a sheet resistance equal to or lower than 1 kxcexa9/square. In practice it appeared that, with such a value, a good shielding of the occurring alternating electric field generated by the deflection unit is obtained. The notion sheet resistance (expressed in xcexa9/square) is explained on page 113 of the book xe2x80x9cDevice Electronics for Integrated Circuitsxe2x80x9d, by R. S. Muller and Th. I. Kamins (2nd edition, John Wiley and Sons).
These and other aspects of the invention will be elucidated with reference to the embodiments described hereinafter.